1. Technical Field
The present invention relates to a method of arc welding using a welding wire, which is a consumable electrode.
2. Background Art
General welding processes used to weld two zinc coated steel sheets together include short-circuit transfer welding (such as CO2 welding and MAO welding) and pulse MAO welding. FIGS. 6, 7A, and 7B show a conventional method of arc welding in the case of welding two zinc coated steel sheets together. FIG. 6 is a sectional view of a bead formed when two zinc coated steel sheets are welded together by general consumable electrode arc welding. FIGS. 7A and 7B show a conventional method of arc welding; FIG. 7A shows changes in welding current with time, and FIG. 7B shows changes in wire feed speed with time.
Zinc coated steel sheet 103 and zinc coated steel sheet 104 have zinc plating 110 on their surfaces. Zinc has a boiling point of 907 degree Celsius, which is lower than the melting point of iron (1536 degrees Celsius). Consequently, when zinc coated steel sheets 103 and 104 are arc welded together, the zinc vaporizes and the vaporized zinc (hereinafter, “zinc vapor”) tries to diffuse to the outside through the melt pool. When however, the molten metal has a high rate of solidification, the zinc vapor is not completely diffused outside, and remains as pores 120 inside and on the surface of weld bead 107. Pores 120 are referred to as blowholes when remaining inside weld bead 107 and as pits when opening in the surface weld bead 107. Since these blowholes and pits degrade the strength of the welded joint, it is essential to suppress their generation, for example, in the automotive industry which uses a large number of zinc coated steel sheets. Pit generation, in particular, is often specified and controlled.
As shown in FIGS. 7A and 7B, assume that pulse welding is performed using either Ar (argon) or a mixture gas of Ar and 25% or less of carbon dioxide. In this case, it is known to use a method of arc welding as shown in FIG. 7A (see, for example, Japanese Unexamined Patent Publication H06-285643). In FIG. 7A, a welding current Aw is applied for a waving period TW consisting of a first period TL and a second period TH in one cycle, and a wire is fed at a wire feed speed Wf shown in FIG. 7B. As shown in FIG. 7A, in the waving period TW, the first period TL has a current waveform where a first average arc force FL acts on the melt pool. In the first, period TL, a peak current IL is fed for a duration tL at a pulse period TP. The second period TH has a current waveform where a second average arc. force FH larger than the first average arc force FL acts on the melt pool. In the second period TH, a peak current IH is fed for a duration tH at the pulse period TP. The waving frequency TW in this case fluctuates in the range of 10 Hz to 50 Hz.
An arc force acts to push down the melt pool. Therefore, when an arc force fluctuates between the first average arc force FL and the second average arc force FH, the melt pool ripples. When the melt pool is rippling, pores 12 generated in the zinc-plated layer reach the surface of the melt pool due to their buoyancy and the flow of the melt pool, thereby being released to the outside of the melt pool.
The conventional method of arc welding described above with reference to FIGS. 7A and 7B shows the analysis to reduce blowholes and the results as an Example with zinc coated steel sheets having a thickness of 1.6 mm and a zinc weight of 45 g/m2. The main purpose, of this method, however, is to vibrate the melt pool, and it is impossible to move the melt pool to an extent to expose root area 121 (shown in FIG. 6) where the steel sheets are overlapped with each other. Consequently, when the sheet thickness is 2.0 mm or more, the required penetration increases, causing an increase in the thickness of the melt pool, and making it difficult to release the zinc vapor. When two welding zinc coated steel sheets having a zinc weight larger than 45 g/m2 are welded together, the amount of zinc vapor itself increases. The zinc vapor remains in weld bead 107, causing an increase in the generation of pores 120.
When the zinc vapor comes up from the melt pool and is released through the surface of the pool, the molten metal may spatter to the outside directly. The molten metal spouted by a release of zinc vapor may also be short-circuited to the wire to generate electric energy, which causes the molten metal to scatter, resulting in an abnormally large number of spatters.